1. Field of the Invention
The field of the invention is that of acoustic transducers that can be used especially in medical or underwater imaging.
2. Discussion of the Background
In general, an acoustic probe comprises a set of piezoelectric transducers connected to an electronic control device by means of an interconnection array.
These piezoelectric transducers emit acoustic waves which, after reflection in a given medium, provide information on said medium. Generally, one or more acoustic matching plates, for example of the quarter-wave type, are attached to the surface of the piezoelectric transducers to improve the transfer of acoustic energy in said medium.
These matching plates may be made out of a polymer type material charged with mineral particles whose proportions are adjusted to obtain the desired acoustic properties. In general, these plates are shaped by moulding or machining and then joined by bonding to one of the faces of the piezoelectric transducers.
More specifically, in the case of a probe possessing a set of elementary transducers, the piezoelectric transducers are separated mechanically by a cutting up of a monolithic plate of piezoelectric material, for example PZT type ceramic. It is then also necessary to cut out the associated acoustic matching layer or layers in the same way so as to avoid any acoustic coupling between elementary transducers through this matching layer or layers. The cutting out of these matching layers and of the piezoelectric layer is therefore generally done simultaneously, for example by means of a diamond-tipped saw.
Each elementary piezoelectric transducer must be connected on the one hand to the ground and on the other hand to a positive contact (also called a hot point).
In general, the ground is located towards the propagation medium (for example the patient in the case of an acoustic echography probe), namely it should be on the side where the acoustic matching elements are positioned.
The simultaneous cutting out of acoustic matching layers and of piezoelectric material has the consequence wherein the ground electrode too is cut out when this electrode is constituted by a metal layer inserted between the acoustic matching material and the piezoelectric material. In the case of a one-dimensional array probe the continuity of the ground electrode is preserved in one direction. In the case of a two-dimensional array probe, where the elements are cut out in both directions, the continuity of the ground electrode must be preserved in at least one direction so as to enable the retrieval of the ground at the periphery of the matrix assembly of elementary piezoelectric transducers.
In the prior art, in order to preserve a continuity of the ground in the case of a two-dimensional probe, it has been proposed to proceed as follows:
On the interconnection array 1, a conductive layer is deposited and then a plate of piezoelectric material is deposited by bonding.
Successive cutting-out operations are performed, in a direction Dy illustrated in FIG. 1, on the matrix of transducers Tij. One or more acoustic matching plates are bonded in the same way. The lower face of the first acoustic matching plate is metallized, enabling the grounds to be brought to the edges of the matrix.
Finally, the entire unit (acoustic matching plates and piezoelectric material plate) are cut out in the direction Dx perpendicular to the direction Dy.
There is thus obtained a matrix of elementary piezoelectric transducers Tij covered with acoustic matching elements Ai, with ground electrodes Pi inserted between the transducers Tij and the elements Ai.
However, this method has the drawback of mechanically connecting the elementary transducers of one and the same line i in the direction Dx, and is therefore detrimental to the performance characteristics of the acoustic probe that results therefrom.
This is why the invention proposes an acoustic probe comprising a continuous ground electrode inserted between elementary piezoelectric transducers uncoupled from one another, and acoustic matching elements also uncoupled from one another so as to resolve the problem of the prior art.
More specifically, an object of the invention is an acoustic probe comprising acoustic matching elements, elementary piezoelectric transducers and an array of interconnections connecting the acoustic transducers to an electronic signal processing and control device characterized in that said probe comprises a continuous ground electrode inserted between the elementary acoustic transducers and acoustic matching elements.
The ground electrode may typically be a metal foil, for example made of copper or silver.
It may also be a metallized polymer film of the copper-plated or gold-plated polyester or polyimide type, or again a polymer film charged with conductive particles.
The acoustic matching elements may advantageously be made of epoxy resin charged with tungsten and/or aluminium oxide particles while the elementary piezoelectric transducers may be made of PZT type ceramic.
According to one variant of the invention, the acoustic probe comprises acoustic matching elements Aij1, with an impedance close to that of the propagation medium of the acoustic probe, that are located above the ground electrode and acoustic matching elements Aij2, with an impedance close to that of the piezoelectric transducers, that are located between the ground electrode and the piezoelectric transducers.
Typically, when the acoustic probe according to the invention is designed to work in an aqueous medium, the piezoelectric transducers being made of ceramic, the elements Aij1 have an impedance of about 2 to 3 Mega Rayleigh and the elements Aij2 have an impedance of about 8 to 9 Mega Rayleigh.
An object of the invention is also a method for the manufacture of the acoustic probe according to the invention. This method comprises the making of elementary piezoelectric transducers (Tij) on the surface of an array of interconnections connecting the acoustic transducers to an electronic signal processing and control device characterized in that it furthermore comprises the following steps:
the depositing of a conductive layer that constitutes an ground electrode (P) on the surface of the elementary transducers (Tij);
the depositing of at least one layer of acoustic matching material;
the selective etching of the layer or layers of acoustic matching materials with a corrosion barrier on the conductive layer so as to constitute acoustic matching elements (Aij).
Advantageously, the selective etching may be done by a CO2 type laser, an excimer type ultraviolet laser or else a YAG type laser.
According to one method of manufacture of the acoustic probe of the invention, the ground electrode may be a metallized copper-coated polyimide film, and the acoustic matching elements Aij may then be defined by the etching, with a CO2 laser at an energy density in the range of some Joules per cm2 (so as not to corrode the metallization), of a layer of epoxy resin charged with tungsten particles.
According to one variant of the method of the invention, two layers of acoustic matching material are deposited, a first layer having an impedance close to that of the piezoelectric transducers and a second layer having an impedance close to that of the medium in which the acoustic probe is designed to function. The set of two layers is etched with a corrosion barrier on the conductive layer.
According to another variant of the invention, a layer that has impedance close to that of the transducers and is conductive is deposited on the surface of a layer of piezoelectric material, the unit is cut out so as to define the piezoelectric transducers Tij and a first series of high-impedance acoustic matching elements. A conductive ground electrode layer is deposited on the set of transducers Tij covered with the elements Aij1. A second acoustic matching layer is placed on the surface of the ground electrode P, elements Aij2 are then defined by the selective cutting out of the low-impedance layer with an etching barrier on the ground electrode.